With a motor vehicle automatic HVAC control, the operator sets a desired temperature for the vehicle cabin, and an electronic control module controls the blower speed, air discharge temperature and air delivery mode based on the set temperature and a number of parameters including outside air temperature, solar loading, and a measure of the actual air temperature in the cabin.
A typical prior art control is illustrated by the block diagram of FIG. 1A, the calculation diagram of FIG. 1B, and the control output diagram of FIG. 1C. As indicated in FIG. 1A, a calibrated constant K and a number of terms based on the ambient or outside air temperature (Tamb), solar loading (Tsolar), desired temperature setting (Tset), and in-car temperature (Tin-car) are developed from sensor information and calibrated gains F1-F4, and combined at summing junction 10 to form a Program Number. The Program Number is applied to a Table 12 which outputs control settings for blower speed (BLOWER), air discharge temperature (DISCHARGE) and air delivery mode (MODE). The control settings are applied to the HVAC controller 14, which in turn, is connected to the CABIN 16 by a number of air delivery ducts 18. The HVAC controller 14 measures the actual air discharge temperature with one or more duct temperature sensors 20, and controls a refrigerant compressor and temperature mix doors (not shown) to satisfy the commanded discharge temperature(s). Such controls, in turn, influence the temperature in the CABIN 16, which is detected by the in-car temperature sensor 22. As indicated in FIG. 1B, the Tin-car, Tsolar and Tamb terms are considered as corrections, and oppose the sum of K and the Tset terms in computing the Program Number. As indicated on the horizontal axis of FIG. 1B, increasingly lower Program Numbers correspond to increased cooling demand, and increasingly higher Program Numbers correspond to increased heating demand. FIG. 1C illustrates in general how the Table 12 derives the control settings for BLOWER, DISCHARGE and MODE, based on the Program Number. In the illustration, MODE comprises two control settings: one for air inlet source (RECIRC or OUTSIDE) and one for air outlet location (PANEL, BI-LEVEL or FLOOR). The blower speed (BLOWER) varies between high and low settings, as does the air discharge temperature (DISCHARGE).
While the outside air temperature and the solar loading can be determined with a fair degree of accuracy, the cabin (in-car) temperature is difficult to accurately determine because the temperature sensing element has to be hidden and aspirated with cabin air, and typically ends up being installed in a less than ideal location from a sensing performance standpoint. For these and other reasons, in-car temperature measurements frequently exhibit a lagging response time and drift, which can degrade the performance of the overall control system, possibly requiring repeated operator adjustment of the temperature setting in order to achieve the desired comfort level. Furthermore, in-car temperature sensing significantly adds to the hardware, calibration and installation cost of the system.
Accordingly, it has been proposed to eliminate the conventional in-car temperature sensor. For example, the U.S. Pat. No. 5,810,078, issued Sep. 22, 1998, discloses a control system which calculates a theoretical in-car temperature by solving a coupled set of ordinary differential equations, and then adjusts the system control variables in closed-loop fashion based on the deviation of the calculated in-car temperature from the desired in-car temperature. However, such an approach necessitates additional sensors, is subject to the usual closed-loop stability concerns, and requires a relatively high degree of computational capability by the system controller.
Accordingly, what is desired is a control system which eliminates the conventional in-car temperature sensor and the performance degradation due to its lagging response time and drift, while not increasing the overall cost and complexity of the system.